The present invention relates to a magnetic recording tape that is excellent in durability, particularly in a cyclic environment, and a method for manufacturing the same, and more particularly it relates to a magnetic recording tape that has suppressed ununiform elongation of the tape and thus can record and reproduce data with high reliability even after repeated use under a cyclic environment, and a method for manufacturing the same.
As means for increasing a capacity of a magnetic recording medium, such as a tape, there has been a method of decreasing a thickness of a substrate. When the thickness of a substrate is decreased, however, there are cases where such a phenomenon occurs in that the medium is ununiformly elongated in the longitudinal direction due to a driving tension on recording and reproducing to cause turbulent winding or decrease in running stability, whereby accurate recording and reproduction of data cannot be achieved. The phenomenon becomes conspicuous upon storage and use under high temperatures, under which thermal contraction is liable to occur. The phenomenon brings about a serious problem in a field of a recording medium for backing up data, in which the reliability of data recording and reproduction is extremely important. There is thus demanded to develop a highly reliable magnetic recording medium having high durability that does not cause ununiform elongation of the medium even under severe environments.
Therefore, an object of the invention is to provide a magnetic recording tape having improved durability capable of recording and reproducing data with high reliability.
Another object of the invention is to provide a method for manufacturing the magnetic recording tape.
As a result of various investigations for shape stability of magnetic recording media conducted by the inventors, a magnetic recording medium that attains the object can be obtained by such a manner that a creep deformation ratio of the medium measured under a specific condition is set less than a specific value.
The invention has been made based on the above finding and relates to a magnetic recording tape which comprises a flexible substrate having a thickness equal to or less than 7 xcexcm and at least one magnetic layer provided on one surface of the substrate, the magnetic recording tape having a longitudinal creep deformation ratio in a range of less than 0.04% when the magnetic recording tape is subjected to a longitudinal tensile stress of 19.1 MPa for 25 minutes at 50xc2x0 C.
The invention also relates to a method for manufacturing a magnetic recording tape comprising a flexible substrate having a thickness equal to or less than 7 xcexcm and at least one magnetic layer provided on one surface of the substrate, the method comprising subjecting said magnetic recording tape to a heating treatment at such a temperature (T) below the equilibrium glass transition temperature (ETg) of the substrate that (ETg-T) is from 1 to 40xc2x0 C. for 0.5 to 240 hours, thereby obtaining the magnetic recording tape having a longitudinal creep deformation ratio in a range of less than 0.04% when the magnetic recording tape is subjected to a longitudinal tensile stress of 19.1 MPa for 25 minutes at 50xc2x0 C.
The magnetic recording tape of the invention will be described in detail with reference to preferred embodiments thereof below.
In one embodiment of the magnetic recording tape of the invention, a magnetic or non-magnetic lower layer is provided on one surface of a substrate, and a magnetic layer as an uppermost layer (hereinafter simply referred to as an upper layer) is provided on the lower layer. A back layer is provided on another surface of the substrate.
In the magnetic recording tape of the invention, a substrate having a thickness equal to or less than 7 xcexcm, preferably a thickness of from 2 to 7 xcexcm, and more preferably from 2 to 4.5 xcexcm, is used. In other words, the magnetic recording tape of the invention is one of an extremely thin type. When a thin magnetic recording tape is subjected to recording and reproduction on a drive, the tape causes ununiform elongation due to a driving tension to deteriorate the running stability. However, the magnetic recording tape of the invention, owing to the creep deformation ratio of less than the specific value measured under the specified condition, causes no ununiform elongation even when the medium is stored or used under severe environments, so as to improve the durability, particularly cyclic durability, of the medium. As a result, recording and reproduction of data can be achieved with high reliability.
When the creep deformation ratio is less than 0.04%, and preferably less than 0.038%, ununiform elongation of the medium is effectively prevented. It is more preferred as the creep deformation ratio is smaller and is ideally zero, but the lower limit thereof that can be attained by the current level of technology is about 0.012%.
While the standard relating to the creep deformation ratio of a magnetic recording tape is defined in Japanese Industrial Standard(JIS) as remaining elongation (JIS X6141), the remaining elongation defined in JIS is the one measured under considerably mild conditions but not under the severe conditions as in the invention. Therefore, even a tape satisfying the JIS standard cannot always attain the object of the invention. The invention is characterized by the fact that when the creep deformation ratio measured under the specified conditions becomes less than the specified value, no ununiform elongation occurs when the tape is stored or used under severe conditions, independent as to whether or not the tape satisfies the JIS standard. When the creep deformation ratio of the invention is measured according to the JIS standard, no result of significant difference can be obtained. Therefore, the creep deformation ratio of the invention is measured by the method described in detail in Examples described later.
While a method for obtaining the creep deformation ratio in the specified range is not particularly limited, it has been found effective to subject the tape to a heating treatment at a prescribed temperature for a prescribed period of time.
The temperature of the heating treatment is preferably lower than the equilibrium glass transition temperature (hereinafter referred to as ETg) of a material constituting the substrate of the magnetic recording tape by from 1 to 40xc2x0 C., more preferably from 1 to 35xc2x0 C., further preferably from 1 to 25xc2x0 C., and particularly preferably from 1 to 20xc2x0 C. When the heating treatment temperature is lower than the ETg of the material constituting the substrate by more than 40xc2x0 C., the heating treatment time becomes too long. When the heating treatment temperature is higher than the ETg, there are cases where the creep deformation ratio cannot be less than the specified value. The ETg is herein defined as follows. A peak temperature of loss modulus of the substrate measured in a frequency range of from 0.0016 to 16 Hz is plotted against reciprocals of the frequency and the temperature to form an Arrhenius plot that is obtained in such a manner that the support is subjected to a suitable heating treatment to make the Arrhenius plot being the Williams-Landel-Ferry(WLF) type (M. L. Williams, et al., J. Am. Chem. Soc., vol. 77, p. 3710 (1955)), and the Arrhenius plot is extrapolated to the lower frequency side to obtain a limiting temperature, which is designated as an ETg.
The specific temperature of the heating treatment varies depending on the material constituting the substrate, and for example, in the case of polyethylene terephthalate (hereinafter referred to as PET) having an ETg of 95xc2x0 C., the heating treatment temperature is preferably from 55 to 94xc2x0 C., more preferably from 70 to 94xc2x0 C., and most preferably from 75 to 94xc2x0 C.
The period of time for the heating treatment is, while depending on the heating temperature, preferably from a several minutes to 240 hours, more preferably from 10 minutes to 240 hours, and particularly preferably from 30 minutes to 240 hours. When the period of time for the heating treatment is too short, there are cases where the effect of the heating treatment cannot be stably exhibited. Even when the period exceeds 240 hours, the effect cannot further advance by prolonging the period, and thus it is not preferred from the standpoint of production efficiency. After the heating treatment, the tape is preferably subjected to annealing to room temperature.
In the case where PET is used as the substrate, the creep deformation ratio can be easily less than the specified value by subjecting the substrate to the heating treatment in such a manner that a peak temperature of loss modulus of the substrate measured at 0.0016 Hz, which is related to micro-Brownian motion of polymer main chains located in non-crystalline regions, is lower than the value of an equilibrium state of the substrate by not more than 8xc2x0 C., and preferably not more than 7xc2x0 C. The equilibrium state herein means such a state that an Arrhenius plot obtained by plotting a peak temperature (relaxation temperature) of loss modulus against reciprocals of the frequency and the temperature becomes the WLF type in the range of frequency of from 0.0016 to 16 Hz. For PET, such state can be obtained by heat treatment for 7 days at 5xc2x0 C.
The material constituting the substrate may be a polymeric material having no mechanical relaxation peak (peak on the temperature dependency of loss modulus) in a range of frequency of from 0.0016 to 16 Hz and a range of temperature of from 10 to 60xc2x0 C. Specific examples thereof include PET and polyphenylene sulfide, but it is not limited to them. The substrate constituted with the material may be subjected to uniaxial or biaxial orientation. The material may be used singly or in combination of two or more kinds thereof. The substrate constituted with the material may be subjected to a corona discharge treatment depending on necessity. Among the materials, PET is particularly preferably employed from the standpoints of film forming property, strength and cost.
The lower layer provided on the substrate contains various kinds of powder dispersed in a binder. Examples of the powder used include non-magnetic powder, an abrasive and carbon black. In the case where the lower layer is a magnetic layer, various kinds of ferromagnetic powder, such as hexagonal barium ferrite, is mixed in addition to the powder. The lower layer can be formed by coating and drying a lower layer coating composition obtained by dispersing these components in a solvent. As for the binder, the non-magnetic powder, a lubricant, the abrasive, the carbon black and other additives, those known in the art can be employed. Specific examples thereof include the binders described in Japanese Patent Laid-Open No. 35246/1997, column 4, lines 25 to 32, and the non-magnetic powder described in Japanese Patent Laid-Open No.35246/1997, column 9, line 44 to column 10, line 24.
The dry thickness of the lower layer is preferably from 0.2 to 3.0 xcexcm, and more preferably from 0.5 to 2.5 xcexcm, from the standpoints of prevention of deterioration in calendering characteristics of the upper layer, improvement in adhesion property with the upper layer, improvement in impact resistance and durability of the tape, and prevention of excess cupping of the tape.
The upper layer provided on the lower layer contains ferromagnetic powder dispersed in a binder. The ferromagnetic powder preferably has an acicular or spindle shape, by which the surface smoothness can be easily obtained. Examples of the ferromagnetic powder having such shapes include ferromagnetic metallic powder and ferromagnetic iron oxide powder. As the binder of the upper layer, those contained in the lower layer may be employed. The upper layer may contain those component described for the lower layer, as components in addition to the ferromagnetic powder and the binder. The upper layer can be formed by coating and drying an upper layer coating composition containing the components dispersed in a solvent.
The dry thickness of the upper layer is preferably from 0.02 to 2.5 xcexcm, more preferably 0.02 to 2.2 xcexcm, and particularly preferably from 0.05 to 1.8 xcexcm, from the standpoints of improvement in reproduction output on recording short frequencies, improvement in surface property and orientation property, and accomplishment of a high S/N ratio.
A back layer provided on the back surface of the substrate is constituted with a single layer or plural layers. Examples of the single back layer include known ones used in magnetic recording media. Specifically, one described in Japanese Patent Laid-Open No. 35246/1997, column 5, line 41 to column 9, line 4 may be employed. The thickness of the back layer is preferably from 0.05 to 0.8 xcexcm, and more preferably from 0.1 to 0.7 xcexcm.
While the embodiment of the magnetic recording tape of a multi-layer coating type has been described heretofore, at least one of the upper layer, the lower layer and the back layer may be a continuous thin film formed by a vacuum film forming method, such as vapor deposition and sputtering. However, since the invention exhibits a conspicuous effect when the coated layer of the coating type tape is thin, it is preferably applied to a multi-layer coating type tape.
The magnetic recording tape of the invention can be used in all recording and reproduction systems of a rotation head type, such as a helical scanning head, and a fixed head type, and exhibits a conspicuous effect when it is used in a recording and reproduction system of a fixed head type.